Traditionally, cobalt-based alloys, including Stellite alloys, have been used for wear-based applications including, for example, in nuclear power applications. The alloys may be used to both form components or to provide hard-facing where harder or tougher material is applied to a base metal or substrate.
It is common for hard-facing to be applied to a new part during production to increase its wear resistance. Alternatively, hard-facing may be used to restore a worn surface. Extensive work in research has resulted in the development of a wide range of alloys and manufacturing procedures dependent on the properties and/or characteristics of the required alloy.
Within the nuclear industry the presence of cobalt within an alloy gives rise to the potential for the cobalt to activate within a neutron flux to result in the radioisotope cobalt-60 which has a long half-life. This makes the use of cobalt undesirable for alloys used in this industry. The cobalt may be released as the alloy wears through various processes, one of which is galling that is caused by adhesion between sliding surfaces caused by a combination of friction and adhesion between the surfaces, followed by slipping and tearing of crystal structure beneath the surface. This will generally leave some material stuck or even friction welded to the adjacent surface, whereas the galled material may appear gouged with balled-up or torn lumps of material stuck to its surface.
Replacements for Stellite have been developed by the industry with low or nil cobalt quantities. Exemplary alloys are detailed in the table below:
AlloyCrCNbNb + VaNiSiFeCoTiGB216708815-251-35-155-152.7-5.6BalNilNilT518319-221.8-2.26.5-8.08.5-10.5 4.5-5.25Bal0.2TraceU.S. Pat.19-221.7-2.08.0-9.08.5-10.55.25-5.75Bal0.20.3-0.7No. 5,660,939
In GB2167088 niobium is provided, but always with the presence of vanadium, which prevents the chromium from combining with the carbon and weakening the matrix. The vanadium also acts as a grain refiner within the wholly austenitic alloy that helps the keep the size of the grains within the alloy within an acceptable range.
The alloys of U.S. Pat. No. 5,660,939 modified the alloy of T5183 by the deliberate addition of titanium and by increasing the amounts of niobium and silicon. The controlled additions of titanium, niobium and silicon alter the structure of the steel to provide a duplex austenitic/ferritic microstructure which undergoes secondary hardening due to the formation of an iron silicon intermetallic phase.
Further hardening is achievable by hot isostatic pressing (HIPPING) of the stainless steel alloy when in powder form where secondary hardening occurs within the ferritic phase of the duplex microstructure.
The niobium provides a preferential carbide former over chromium, enabling high chromium levels to be maintained within the matrix so as to give good corrosion performance. Low cobalt based alloys, or cobalt alloy replacements, typically comprise significant quantities of carbide forming elements which can form alloys with hardness values in excess of 500 Hv. As with traditional Stellite alloys, the high levels of hardness observed can make machining difficult, resulting in poor mechanical properties for, for example, ductility, fracture toughness, impact resistance and workability. Additionally, the cost of using such alloys is high due to the need for special treatments and/or precision casting or other near net shape manufacturing methods to limit further machining.
Accordingly, it would therefore be advantageous to provide an alloy without the aforementioned disadvantages.